跟着官方文档学HINT（三）优化器特性

SQL> show parameter optimizer_features_enable

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
optimizer_features_enable            string      11.2.0.3

OPTIMIZER_FEATURES_ENABLE =
{ 8.0.0 | 8.0.3 | 8.0.4 | 8.0.5 | 8.0.6 | 8.0.7 | 8.1.0 | 8.1.3 | 8.1.4 | 8.1.5 | 8.1.6 | 8.1.7 |
9.0.0 | 9.0.1 | 9.2.0 | 9.2.0.8 |
10.1.0 | 10.1.0.3 | 10.1.0.4 | 10.1.0.5 | 10.2.0.1 | 10.2.0.2 | 10.2.0.3 | 10.2.0.4 | 10.2.0.5 |
11.1.0.6 | 11.1.0.7 | 11.2.0.1 |11.2.0.2  |11.2.0.3 |11.2.0.4 }

SESSION  SYSTEM 级别可改

也可以使用本文推荐的提示 optimizer_features_enable  顾名思义就是在某条sql语句中声明优化器使用的版本

下面做一个实验看一下

SQL> create table test_skip_scan
2  as
3  select mod(level,10) low_dist ,level uniq
4  from dual
5  connect by level <=1000000;

Table created.

SQL> create index IDX_DAO_SKIP on TEST_SKIP_SCAN (LOW_DIST, UNIQ);

Index created.

SQL> ANALYZE TABLE  test_skip_scan  COMPUTE STATISTICS FOR ALL INDEXED COLUMNS;

Table analyzed. 

SQL> select * from test_skip_scan where uniq=888 ;

Execution Plan
----------------------------------------------------------
Plan hash value: 1582535247

---------------------------------------------------------------------------------
| Id  | Operation        | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |              |     1 |     6 |    12   (0)| 00:00:01 |
|*  1 |  INDEX SKIP SCAN | IDX_DAO_SKIP |     1 |     6 |    12   (0)| 00:00:01 |
---------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("UNIQ"=888)
       filter("UNIQ"=888)

Note
-----
   - dynamic sampling used for this statement (level=2)

Statistics
----------------------------------------------------------
          6  recursive calls
          0  db block gets
        100  consistent gets
         19  physical reads
          0  redo size
        596  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

select /*+ optimizer_features_enable('8.0.0') */  * from test_skip_scan where uniq=888 ;

Execution Plan
----------------------------------------------------------
Plan hash value: 2650644146

--------------------------------------------------------------------
| Id  | Operation         | Name           | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT  |                |     1 |    26 |   277 |
|*  1 |  TABLE ACCESS FULL| TEST_SKIP_SCAN |     1 |    26 |   277 |
--------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("UNIQ"=888)

Note
-----
   - cpu costing is off (consider enabling it)

Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       1788  consistent gets
          0  physical reads
          0  redo size
        596  bytes sent via SQL*Net to client
        523  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

由于使用提示指定优化器版本为8.0.0 ，而index skip scan 又是Oracle9i 引入。
所以生成的执行计划必然不会使用，执行计划变成了全表扫描。